Image intensifier tubes are used in night/low light vision applications to amplify ambient light into a useful image. A typical image intensifier tube is a vacuum device, roughly cylindrical in shape, which generally includes a body, photocathode and faceplate, microchannel plate (“MCP”), and output optic and phosphor screen. Incoming photons are focused on the glass faceplate by external optics, and strike the photocathode which is bonded to the inside surface of the faceplate. The photocathode or cathode converts the photons to electrons, which are accelerated toward the input side or electron-receiving face of the MCP by an electric field. The MCP has many microchannels, each of which functions as an independent electron amplifier, and roughly corresponds to a pixel of a CRT. The amplified electron stream emanating from the output side or electron-discharge face of the MCP excites the phosphor screen and the resulting visible image is passed through the output optics to any additional external optics. The body holds these components in precise alignment, provides electrical connections, and also forms the vacuum envelope.
Conventional MCPs are formed from the fusion of a large number of glass fibers, each having an acid etchable glass core and one or more acid-resistant glass cladding layers, into a solid rod or boule. Individual plates are sliced transversely from the boule, polished, and chemically etched. The MCPs are then subjected first to a hydrochloric acid bath that removes the acid etchable core rod (decore), followed by a hot sodium hydroxide bath that removes mobile alkali metal ions from the glass cladding.
Detection and amplification of low-level image signals is a critical function in a wide variety of military and civilian applications. Many high-gain detectors, numerous types of photomultiplier tubes, and most image intensifier tubes incorporate MCPs as the primary amplifying device. The diverse fields in which MCP-based systems are used today include military uses (for e.g., night vision devices, weapon sights, aerospace vision systems) and scientific uses (for e.g., electron microscopes, fast oscilloscopes, X-ray images amplifiers, field-ion microscopes, time-correlated photon counters, quantum position detectors).
Additional applications of MCPs include astronomical uses (for e.g., grazing-incidence telescopes for soft X-ray astronomy, concave grating spectrometers for exploration of planetary atmospheres, laser satellite ranging systems), medical uses (for e.g., observation of low-level fluorescence and luminescence in living cells, radio luminescence imaging, correction of night blindness), and commercial uses (for e.g., night vision consumer products for security and law enforcement, search and rescue operations, outdoor sport and recreation).
Although ITT Night Vision, along with several other manufacturers, has recognized the strategic importance of moving towards new, dynamic markets and is branching into consumer products, the military market remains dominant. Prices well over $500 for a low-end night vision product constrain expansion into more cost-conscious, non-military markets. The commercial market for consumer products based on MCPs is currently very small, and night vision has remained an expensive luxury that is out of reach for most individuals.
The commercial sector of MCPs is hugely underdeveloped: if MCPs were available at reasonable prices, such as under $100, or even under $50, they could become the basis of a vast number of popular consumer products with market size in billions of dollars. From simple night vision goggles or glasses for night-time drivers (particularly the elderly and sight-impaired), hunters, boaters, night time divers, and even dog-walkers, to more advanced devices for search and rescue, night-time filming, CCTV surveillance and security systems, the range of possible applications is immense. A significant reduction in the price of MCPs is the only way to open up these huge, untapped markets.
The current process used in industry for manufacturing microchannel plates is primarily based on the technology of drawing glass fibers and fiber bundles. Referring now to FIGS. 1–4, the multiple conventional processing steps required for manufacture of MCPs are illustrated. Referring to FIG. 1, there is shown the beginning step of the fabrication process. It will be understood that FIG. 1 is not drawn to scale, especially with regard to the longitudinal axis of the tube 200. The fabrication process begins with tubes 200 of specially formulated glass, usually lead oxide glass 210, that is optimized for secondary electron emission characteristics. Solid cores 220 of a second glass with a different etching characteristic are inserted into the tubes. The filled tubes are softened and drawn to form a fiber, as shown in FIG. 1. Referring to FIG. 2, the next fabrication step involves combining millions of such fibers together in a bundle 300 in a hexagonal close-packed arrangement. The bundle is fused together at a temperature of 500° C.–800° C. and again drawn out until the solid core diameters become approximately equal to the required channel diameter (40 to 10 μm, see FIG. 3). Referring to FIG. 3, individual microchannel plates 240 are cut from this billet 400 by slicing at the appropriate bias angle to the billet axis, as shown in FIG. 3. The thickness of the slice is generally chosen to give a channel length-to-diameter ratio of 40–80.
The individual plates are next ground and polished to an optical finish. The solid cores are removed by chemical etching in an etchant that does not attack the lead oxide glass walls, thus generating hollow channels through the plates. Further processing steps lead to the formation of a thin, slightly conducting layer beneath the electron-emissive surface of the channel walls. Referring to FIG. 4, there is shown a cross-section and side view of a resulting wafer of microchannel plates. Electrodes 260, in the form of thin metal films, deposited on both faces of the finished wafer. A thin membrane of SiO2 (formed on a substrate which is subsequently removed) is deposited on the input face to serve as an ion-barrier film 270. Finally, the plate is secured in one of several different types of flange 280. The finished MCP may now be incorporated into an image-intensification system. As described, the process is very complex and very costly.
Current manufacturing technologies for MCP with materials other than glass also are known. Referring now to FIG. 5, one alternate method of manufacture of MCP with materials other than glass are shown. One of the methods invented to make MCPs with alternate material is by using materials called green sheets. Green sheets are made by first mixing polymer binder and powdered ceramic/glass. This slurry is then coated in sheet form and dried to form green sheets. In the described method, such green sheets were punctured with array of holes of the sizes to MCP tubes. Subsequently, the sheets were stacked on top of each other such that the holes punctured in each sheets align thus forming array of micro tubes, the structure needed for MCP. Subsequently, this whole structure is furnaced at a high temperature to make it solid. It is further processed to provide a gain-enhancing layer on MCP tube surfaces.
In silicon MCPs, an array of holes are etched in silicon wafer using different techniques such as electrochemical etching, reactive ion etching and streaming electron cyclotron resonance etching. This MCP structure in the silicon wafer is then oxidized to form SiO2. It is further processed to provide a gain enhancing layer on channel walls and electrodes on both sides.
The above-described limitations of current MCP manufacturing technology must be overcome. With respect to materials used, very little flexibility is currently available. Constraints in the softening temperature and differential etching characteristics mean that only a few glasses can be used. The material must be doped appropriately to meet the constraints, resulting in the following adverse effects on performance: defects, impurities, nonuniformities, and residues from etching reduce the signal-to-noise ratio and increase energy dispersion. Additionally, low softening temperatures contribute to outgassing, and narrow material spectrum precludes the optimization of secondary emission by means of optimum surface treatment, leading to lower gain and lower saturation voltage.
Resolution depends on the diameter and pitch of the channels as well as the electron energy dispersion, the accelerating voltage, and the distance between the MCP output face and the phosphor surface. Typically, the secondary-electron beam width at the phosphor screen is three times the channel diameter, leading to a very low modulation transfer function (“MTF”) and making focusing necessary. By fusing, drawing, and etching it is impossible or prohibitively expensive to make channel diameters below 4 μm and maintain an open area ratio above 50%. Previous generations of microchannel plates have MTFs well below unity (the ideal MTF is 1 at the channel array pitch) and no dramatic increase is expected from the conventional fiber-drawing technology. The following problems are related to reducing the channel diameter: the walls between the channels become too weak to withstand the subsequent processing steps, especially when the optimum MCP thickness is proportionally reduced to satisfy the constraint L/dc˜40, which leads to poor yield and reduced useful area; etching of narrower channels becomes more difficult; the etching nonuniformity and spatial pattern nonuniformity lead to further increases in noise; the production of large, defect-free areas becomes more difficult; and the treatment of the channels to achieve funneling becomes more difficult.
For MCPs with very small pitch, the conventional manufacturing technology limits the useful area that can be achieved to about 1 square inch (approx. 6 cm2), precluding applications requiring high resolution over large areas.
There have been some alternatives to current glass MCP manufacturing technology.
A method was developed for making a novel microchannel plate (“MCP”) by introducing new materials and process technologies. The key features of their MCP were as follows: (i) bulk alumina as a substrate, (ii) the channel location defined by a programmed-hole puncher, (iii) thin film deposition by electroless plating and/or sol-gel process, and (iv) an easy fabrication process suitable for mass production and a large-sized MCP. Green sheets made up of alumina slurry in binder were punched with a hole puncher into array of holes. Later on many of these sheets were stacked on top of each other with their array of holes aligned to each other and were furnaced to form the MCP structure with circular holes of 170 microns with pitch if 220 microns and thickness of 2 mm. This MCP structure was further processed to make the final MCP structure. The characteristics of the resulting MCP were evaluated with a high input current source such as a continuous electron beam from an electron gun and Spindt-type field emitters to obtain information on electron multiplication. In the case of a 0.28 μA incident beam, the output current enhanced ˜170 times, which is equal to 1% of the total bias current of the MCP at a given bias voltage of 2600 V. When the developers of the process inserted a MCP between the cathode and the anode of a field emission display panel, the brightness of luminescent light increased 3–4 times by multiplying the emitted electrons through pore arrays of a MCP. However, the sizes if the MCP structures made are not suitable for the typical image intensifier tubes.
There have also been other attempts to make MCP structure from GaAs and fused silica using micromachining techniques of dry etching. Etch methods used were magnetron reactive ion etching, chemically assisted ion beam etching (“CAIBE”), and electron cyclotron resonance etching (“ECR”). Extensive characterization of the ECR etcher was carried out with a designed experiment, which used statistical methods to minimize the number of characterization runs. CAIBE gave high aspect ratio etching of GaAs, but at low etch rates. ECR provided higher etch rates of GaAs and better substrate temperature control. The effect of temperature on sidewall roughness and undercut was examined for temperatures as low as −100° C. Features with an aspect ratio greater than 30 are obtained. Etching of fused silica was difficult due to low etch rates (<0.2 μm/min), and faceting of the metal mask.
Other developers have worked on a structure for microchannel plates fabricated using Si micromachining techniques. High aspect ratio pores were constructed using reactive ion etching and streaming electron cyclotron resonance etching, and low-pressure chemical vapor deposition (“LPCVD”). In one process, 40 μm deep pores with 2 μm openings on 4 μm centers were directly etched in Si. Alternatively, pores with aspect ratios of 30:1 were constructed in a low-stress SiNx membrane using a sacrificial template process whereby pillars of Si are etched and then subsequently backfilled with a dielectric using LPCVD. In these micromachining techniques there was no mention of making bias angle in the micro channels needed for the Chevron configuration of MCPs.
Another important technology was developed to make silicon MCPs. After defining a simple lithography step and pre-etch to define the starting channel geometry in a hexagonal pattern on Si wafer, the channels are etched with electrochemical etch. This etch follows the crystallographic plane thus providing any necessary bias angles to the microchannel structure. Typical channels of pitch 8 microns and depth of 350 microns were etched. Further this MCP structure was oxidized and processed to produce final MCP structure. This was characterized electrically to determine the gain of this MCP structure.
Another known method of manufacturing microchannel plates is described in Faris, et al. U.S. Pat. Nos. 5,265,327 and 5,565,729, both entitled “Microchannel Plate Technology,” both of which are fully incorporated by reference herein.
The current manufacturing technology is inherently high-cost due to the numerous processing steps required. A 1-inch diameter MCP with 10 μm pitch has a cost in the range of $500–1000. Accordingly, there remains a need in the art for lower cost MCPs and manufacturing methods that will reduce the cost of MCPs.